In classical vision, retinal ganglion cells relay light information generated by rods and cones. The discovery of a unique ganglion cell population has changed this model: these relays are themselves photoreceptors, and inform diverse, non-image-forming visual processes such as circadian photoentrainment, the pupillary light reflex, and the acute suppression of movement. Little is known about the mechanism of light sensitivity in these neurons. The proposed research will begin by determining how their action potentials are driven by light-stimulated intrinsic and synaptic signals. The intrinsic photocurrent will then be characterized, including its sensitivity to light and voltage, its kinetics, its ionic selectivity, and its potential gating by second messengers. The information will help guide a molecular genetic exploration of the phototransduction cascade. Light is often considered in terms of visual perception, but its widespread importance to physiology is evident in ailments such as seasonal affective disorder and jet lag, where it plays the major role. Understanding the photosensitive ganglion cells will help in understanding basic, light-dependent physiology and its disturbance in illness. [unreadable] [unreadable]